The invention relates to a raster scanner and scanning method, and more particularly to a raster scanning and scanning method wherein the scan line output rate is tailored to the document transport speed to avoid uneven spacing of scan lines.
Raster scanners incorporate a means such as a linear array of photo-optical elements for scanning a document image scan line by scan line. Typically, a platen-like surface is provided on which the document to be scanned is supported, a scanning slit being provided in the platen through which the arrary scans the document. A document transport is provided for moving the document past the scanning slit during the scanning operation to present fresh image areas to the array and preclude scanning of previously scanned areas of the document.
Due to the intrinsic nature of arrays, the pixel output is grouped by scan lines, and scan lines are captured at a rate designed for optimum collection of image information. Unfortunately, the scan line rate of the array seldom corresponds with the demands and needs of the output device or user which the scanner serves, and to accommodate this disparity, a storage buffer is interposed between the array output and the user to store, pending demand, some portion or all of the scan lines produced by the array. Thus, the user can draw scan lines on demand from the buffer at a rate compatible with the user's needs and, as the supply of scan lines in the buffer falls, the scanner is actuated to replenish the buffer. In this type of arrangement, it will be understood that in order to permit the scanner to replenish the buffer in the face of a scan line drawout by the user, the scan line output rate of the scanner must be greater than the rate at which the user draws scan lines from the buffer.
In scanners of the aforementioned type, where the array scans a moving document through a relatively narrow scanning slit, stopping and starting of the document transport in response to demand for scan lines by the buffer is required. Otherwise the buffer may be over-filled if demand for data by the output device is reduced, resulting in the loss of portion of the document image. To avoid this, more than one scan of the document would be required if capture of the entire image is to be assured. However, periodic stopping of the document transport can also lead to duplicate or overscanning problems which come about from the need to maintain common arrays operating continuously. This can result in duplicate scanning of the same line while the document transport is stopped and the document is stationary. And during periods of acceleration and deceleration of the document transport, when the effective speed of the document past the scanning slit is less than the speed required for optimum and undistorted scanning scan lines will be spaced too close together and overlap.